FIG. 1 illustrates a conventional differential amplifier including transistors Q12 and Q14 and a current source Io. Differential inputs applied to the transistors cause the output voltages Vo and {overscore (Vo)} to assume high and low states. It is desirable for the differential amplifier to have a fast switching speed and large bandwidth.
A capacitor C is shown coupled to the emitters of the transistors. This capacitor may be parasitic, caused by line capacitance, collector-base capacitance of Io, base-emitter capacitance of Q12 and Q14, or may be a capacitance intentionally added to the circuit to increase switching speed as described below.
FIG. 2A illustrates the current flowing through resistor R1 when the switching signal into transistor Q12 goes high at time T1. The transient in-rush current is due to the current flowing into the capacitor C in accordance with the equation
  i  =      C    ⁢                            ⅆ          v                          ⅆ          t                    .      
FIG. 2B illustrates the corresponding output voltage during the switching time.
FIG. 3A illustrates the voltage levels of the differential input signals in and {overscore (in)}.
FIG. 3B illustrates the output voltages in response to the switching signals of FIG. 3A without any capacitor C coupled to the emitters or in an ideal environment. Note the switching delay of the output voltages with respect to the input signals. This is due to the switching delays of the transistors.
FIG. 3C illustrates the output voltages when the capacitor C is coupled to the emitters of the transistors. Note that the output voltage, when going high, has a faster response time as compared to the output voltage when going low. When the output voltage goes from low to high, meaning that its associated transistor is turning off due to the switching signal, the fixed current source Io draws some current from the capacitor C, causing less current to flow through the transistors. This reduction in current through the resistors R1 and R2 causes the output voltage to rise more quickly. When the output voltage goes low (such as shown in FIG. 2B), a transient in-rush current through the associated resistor and switching transistor charges capacitor C and initially distorts the output voltage signal until a steady state condition occurs. This distortion is undesirable in applications such as amplifiers where the signal quality is important. The distortion of the falling output signal gets worse when the switching speed of the circuit is increased.
What is needed is an improvement to a switching circuit where increasing switching speed does not result in significant distortion of the output signal.